This invention relates to automatic package wrapping machines and in particular to a machine which wraps packages in polymer film, including heat shrinkable thermoplastic film, and seals the edges of the film to completely encase the packages within the film where the packages move continuously at a high rate of speed through the machine.
In typical prior art bagging and in-line shrink wrap packaging machines, packages are moved towards a cutting and sealing area by an in-feed conveyor. As the packages are moved towards the cutting and sealing area, the packages are surrounded by center folded polymer or shrink wrap film that has been partially unfolded so that the packages may be conveyed between the film layers. Subsequently, the packages are transferred to an exit conveyor where the packages move to a designated sealing and cutting location. At the sealing and cutting location, it is typical to stop the forward movement of the exit conveyor and use a hot knife or hot wire system to longitudinally and laterally seal the layers of polymer film and to sever the film between adjacent packages at the lateral or cross seal. The end result of the cutting and sealing operations are packages that are securely enclosed or “bagged” within polymer film. After the packages are bagged, the exit conveyor is again activated and the packages are typically either packed for shipping or are further conveyed to a shrink wrap tunnel or oven, where the film is heated causing it to shrink over the packages thereby securely wrapping the packages.
One typical prior art machine, commonly referred to as an L-sealer, is described in U.S. Pat. No. 3,583,888, issued to Shanklin, entitled “Packaging Wrapping Apparatus and Method.” An L-sealer uses an L-shaped hot knife to contemporaneously cross-seal and side seal the layers of polymer film when the package is stopped at the cutting and sealing location. L-shaped hot knives are typically coated with TEFLON, so that melted polymer film will not stick to the knife. An advantage of L-sealers is that the packages they make are neat and attractive having trim seals on three sides and clear film on the fourth side and top and bottom. L-sealers, however, have several disadvantages. The dimensions of L-shaped hot knives must be larger than the size of the packages being wrapped. Thus, L-sealers cannot produce longitudinal seals longer than the length of the knife. In addition, it is important that a TEFLON coated hot knife be extremely straight and without nicks. If the knife is improperly adjusted, warped, or nicked, polymer film will not seal properly. For instance, a nick in the knife will create a hole in the seal. Likewise, a warped or improperly adjusted knife will create an incomplete seal.
More recent machines have overcome some of the limitations of L-sealers by using continuous longitudinal or side sealers. These machines have utilized hot wires and ultrasonic welders to create a continuous side seal. However, most of these machines have used a straight hot knife for cross sealing and require the conveyer system to be stopped while the hot knife makes the cross seal. Thus, these machines still suffer from the intermittent operation typical of L-sealer type machines and therefore have the same limitations regarding package throughput.
Efforts have also been made with rotary hot knife systems that can cut and side seal polymer film without stopping the forward movement of the film. Such rotary hot knife side sealing systems typically have a circular TEFLON coated knife mounted for rotation. However, these systems are typically not compatible with some of the thicker polymer films commonly used in high speed applications because of insufficient contact time between the film and the rotary knife to produce suitable welds.
What is needed therefore is a high speed package wrapping machine capable of continuous operation during both side and cross sealing functions. The machine should be readily adaptable for use with a wide range of package sizes without requiring the need to replace hot knives or other hardware to accommodate different size packages. Such a machine would thus be simpler and have a quicker setup time than existing machines. Ideally, such a machine would utilize a sealing mechanism that improves on existing hot knife, hot wire, and ultrasonic sealing devices.